After an oil rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit is typically used to service the well. Servicing includes installing and removing inner tubing strings, sucker rods, and pumps. The variety of work requires a myriad of tools.
One piece of equipment that is found on almost every well service rig is a hoist system for controlling the movement of a cable attached to a traveling block. The hoist winds and unwinds a cable that is attached to a traveling block, which is ultimately used to raise and lower heavy objects, such as rods and tubing, into and out of oil and gas wells. The hoist is usually driven by a variable speed engine coupled in part to the hoist. The prime mover (engine) drives the hoist, usually utilizing a chain driven compound and an air actuated friction clutch, the drum clutch being a critical component of the overall hoist system. The clutch is frequently the most often abused component of the overall drum system. For the most part, the abuse comes from unwanted slippage, which leads to excessive wear on the clutch assembly, leading to a reduced load size the well service rig can lift, and ultimately leading to a total breakdown of the rig.
The coupling effect of a clutch is a function of both the frictional component of the clutch (coefficient of friction and cross sectional area) and the total force between the drum and the compound plates. Naturally, higher hook loads supported by the well service rig require increased coupling between the compound and the drum, thereby requiring a stronger clutch. Since well service rig clutches are commonly air actuated, the amount of air pressure being exerted on the clutch assembly is critical to it operating properly.
The life of a drum clutch on a well service rig varies in accordance to usage and operator technique. On some rigs, clutches can last over 5 years, while on others they are replaced at intervals less than 1 year. This leads to costly rig repairs and even costlier rig down time. Therefore, this industry needs a system to assist the rig operator in operating the drum clutch so as to reduce the wear on the clutch caused by operator technique.